Enforce system radius instead of separate value for widgets in launcher

The change is behind a flag. This is to get to the stage of having
consistent corner radius across rectangular widgets.

Bug: 373351337
Test: Unit test and manual
Flag: com.android.launcher3.enforce_system_radius_for_app_widgets
Change-Id: I23d8d3bb2561b4fc3f780fc4ddce62e8b9293192